Semi-insulating SiC crystals are grown to obtain SiC wafers having a resistivity above 105 ohm-cm for use as substrates in the fabrication of microwave devices. As nitrogen is a known donor impurity in the growth of semi-insulating SiC crystals, it is necessary during said growth to remove nitrogen to very low concentration values, e.g., generally to less than 1016 nitrogen atoms/cm3, in order to achieve acceptable levels of resistivities in the grown SiC crystals.
Since Nitrogen is the major component of the atmosphere, SiC crystals are grown within a high vacuum furnace to remove nitrogen to very low concentration. SiC crystals can be grown by a physical vapor transport (PVT) technique by sublimation of SiC and deposition of the resultant vapor on a single crystal seed of SiC. This process generally occurs at high temperatures in excess of 2000° C. These high temperatures are achieved using an induction-heated graphite susceptor insulated from the furnace wall using sections of porous low-density graphite foam.
A problem with this SiC crystal growth technique is the use of graphite components, including the susceptor, the crucible and heat shields made of dense graphite, as well as thermal insulation made of porous, low-density graphite. Specifically, due to its porosity, graphite is generally capable of absorbing large quantities of gas, e.g., up to 100 cc of air per gram of graphite. Placed inside the growth chamber, graphite is the principle source of nitrogen contamination in the growth of SiC crystals.
Residual nitrogen content inside the crystal growth chamber can be reduced by common and well-known techniques that can be applied prior to growth. Examples of such techniques include: purging the crystal growth chamber with inert gas, or high-temperature heating under high vacuum and by the use of external getters such as heated titanium. Despite using the above techniques, SiC crystals grown in such conditions still contain undesirable levels of nitrogen, especially in the first-to-grow portions of the crystal, that adversely affect the resistivity of SiC wafers obtained therefrom.
The undesirable level of nitrogen in a SiC crystal is due to the desorption of nitrogen from graphite components—a process that occurs at the temperatures of SiC crystal growth. As a result, the nitrogen donor concentration can become comparable or higher than that of the deep-level centers that are responsible for the electrical compensation of the crystal, especially in the first-to-grow portions of the crystal. Wafers sliced from portions of the crystal having too much nitrogen are not fully compensated, are less resistive and, therefore, less useful.